U.S. Department of Energy
Date of this Version
1999
Citation
J. Phys. Chem. B 1999, 103, 5197-5203
Abstract
The dynamics and orientations of interlayer terephthalate (TA) anions in mixed-anion hydrotalcite-like compounds [Mg1-x Alx(OH)2](AyB1-y)x/2•nH2O, where A = p-C6D4(COO)22- and B ) CO32-] were investigated by 2H variable-temperature solid-state nuclear magnetic resonance (NMR) spectroscopy. Two dominant 003 d spacings, 7.6 and 14.2 Å, were observed by X-ray diffraction (XRD) in these samples, suggesting that there were at least two interlayer environments. Over the range of layer charges studied (0.37 ≥ x ≥ 0.29), the intensity of the 7.6 Å XRD peak increased with decreasing x, while the intensity of the 14.2 Å peak increased with increasing x. Analysis of 2H NMR line shapes indicated that the motion of some of the TA anions in every sample could be described as rotational diffusion (RD) about the C-COO- axis at temperatures ranging from 245 to 355 K, with the remainder being essentially immobile. The fraction of TA anions exhibiting rotation (XRD), as estimated by 2H NMR, was positively correlated with the TA content of the interlayer and thus seemed to correspond to the fraction of TA molecules in the 14.2 Å phase. A large difference in XRD was observed between compounds having similar layer charge but different TA mole fractions. At temperatures of ≤ 330 K, XRD increased with temperature. A secondary minimum in XRD, however, was observed for each of the compounds at temperatures between 330 and 350 K. We conclude that the presence of CO32- anions in the interlayer regions, rather than layer charge, is the dominant factor affecting the dynamics and orientation of interlayer TA anions.